1. Field of the Invention
The present invention relates generally to drive means for electrical transducers and, in particular, to an electrically resonant system for responsively driving a rectifying segmented transducer in order to produce the effect of a nonrectifying transducer.
2. Description of the Related Art
A diverse class of electromechanical transducers includes electrostrictive and state-change ferroelectric materials, some magnetic devices including solenoids and related permeable devices. This class produces a mechanical action in the same sense, regardless of the polarity of the applied electrical signal, a production also known as mechanical rectification. Most of the representatives of this class of transducers conduct an amount of electrical energy that is large relative to the electrical energy actually transduced to useful mechanical work, the conducted energy taking the form of capacitive, inductive, or a combination of capacitive and inductive reactance currents. Relatively high electrical efficiency obtains when little of the conducted reactive electrical power is wasted by the electric drive means. By way of example, electrical resonance allows the generation of substantial quantities of reactive energy, often with larger currents and potentials than are otherwise handled by commonly available electronic components. However, electrical resonance is by nature bipolar and essentially symmetric about current and potential average values, the averages usually being zero. Aside from a relatively sparse class of applications that benefit from cyclical rectified mechanical strokes, rectifying transducers are not conveniently made components of an electrically resonant circuit.
Applicant's patent, U.S. Pat. No. 4,928,030, entitled Piezoelectric Actuator issued May 22, 1990, which is hereby made a part hereof and incorporated herein by reference, describes kinds of nonrectifying actuators and transducers that convert electrical signals to forceful mechanical strokes. These transducers generally employ a transducing action in which the mechanical stroke is essentially proportional to the instantaneous amplitude of the applied electrical signal, for example, to the current or to the potential. The proportional response-includes the polarity of the applied electrical signal. The piezoelectric shear embodiments of the taught transducers use bipolar symmetric transduction to obtain a mechanical stroke that is larger than comparable thickness-mode or extension-mode piezoelectric transducers, the latter being generally restricted to monopolar electric drive. Another taught advantage is smooth walking actuation using nonsinusoidal electric drive signals that produce a mechnical walking stroke that forcefully positions an object by traction in a range of positions far exceeding the range provided by a single stroke of the actuator. High mechanical efficiency obtains when the nonsinusoidal electrical and mechanical actions produce the walking without rubbing. However, high electrical efficiency is more difficult to achieve because the inherent high efficiency of electrical resonance is precluded by the exclusion of the sinusoidal electrical wave forms generally accompanying resonance.
Applicant's copending application Ser. No. 07/488,548 filed Mar. 5, 1990 and continued as Ser. No. 07/743,069 filed Aug. 9, 1991 entitled Electric Drive for a Segmented Transducer, which is hereby made a part hereof and incorporated herein by reference, teaches an electric drive means for a segmented transducer, including the transducers described in his Piezoelectric Actuator patent cited above, wherein each segment or subset of segments is stimulated into electrical resonance by a separate circuit. Each subset is made to resonate at a prescribed amplitude, phase, polarity and frequency in accordance with the requirements of the desired nonsinusoidal mechanical stroke, and using the rules of the Fourier Theorem. Unlike conventional Fourier synthesis, wherein a nonsinusoidal waveform is made of components that are electrically summed, the drive means of this invention creates a nonsinusoidal mechanical stroke by mechanical summation of sinusoidal mechanical stroke components. The taught methods apply to transducers that are essentially linear, bipolar, and symmetric in response. Methods of mechanically summing nonsinusoidal stroke wave forms using Fourier synthesis when transducers are rectifying are not known.
Applicant's copending application Ser. No. 07/836,495 entitled Walking Toothed Actuator filed Feb. 18, 1992 which is hereby made a part hereof and incorporated herein by reference, describes a subclass of piezoelectric actuators, herein cited as an example, achieves, by means of engaged teeth, large transducer actuating forces relative to equivalent actuators that rely solely on traction. However, unlike traction actuators, toothed actuators are intolerant of any walking stride that is not an integer of the tooth length period. The toothed class of actuator requires period matching stride length beginning with the first stride of operation, and therefore requires an electrical drive means that provides full stride electrical amplitudes, while also providing an electrical efficiency appropriate to the relatively high electromechanical efficiency of the toothed piezoelectric actuator. It also teaches a method of switching a transducer segment into and out of a resonant electrical circuit in a manner that preserves resonance while immediately bringing the transducer to full operating effectiveness, wherein after initiation of activation, the transducer continues in a bipolar mean-zero operating mode until switched out of the circuit. An electric drive means for the toothed actuator is also described. The taught methods are not applicable to rectifying transducers, except in a sparse class of applications that benefit from rectified mechanical cyclical strokes.
Many piezoelectric electrostrictive materials are rectifying. Cross, L. E. et al. Large Displacement Transducers Based on Electric Field Forced Phase Transitions in the Tetragonal (Pb.sub.0.97 La.sub.0.02) (Ti,Zr,Sn)O.sub.3 Family of Ceramics, J. Appl. Phys., vol 66, (12), Dec. 15, 1989, p 6014-6023, describes one family of compositions of ceramic piezoelectric materials that mechanically rectify an applied electrical drive signal when the intensity of an applied electric field lies in prescribed proximity to the field intensity at which a phase transition occurs. Another well known example of a rectifying transducer is the magnetic solenoid that pulls in one direction regardless of the direction of current flow in the solenoid winding. Another example is electrostriction in which electrostrictive material changes shape in a sense that maintains the same polarity regardless of the direction of the applied magnetic field.